1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method in which printing is performed by ejecting a plurality of pigment-based inks and a treatment liquid for improving the performance of an image formed by the pigment-based inks.
2. Description of the Related Art
In recent years, inkjet printing apparatuses have come to be widely used, for example, in exhibition use for general public and trademark print use such as photos, posters and graphic prints, due to the high resolution and high print quality of an image to be printed. In images formed for the above-described exhibition use for general public and trademark print use, there is an increasing demand for improving image qualities such as gloss uniformity and bronzing, in addition to high resolution and high print quality, and there is also an increasing demand for improving fastness of a printed image which indicates the strength and prolonged storage of an image.
In this instance, the bronzing is to an extent in which illumination light reflects color different from that of the illumination light due to a bronzing phenomenon upon regular reflection (mirror reflection) on the surface of a pigment image. It is known that the phenomenon is in particular apparent when cyan ink is used.
Color inks used in an inkjet printing apparatus are roughly classified into dye-based inks and pigment-based inks. The dye-based inks are characterized in that they are higher in transparency and greater in color development than the pigment-based inks because a color dye is dissolved in a water or alcohol-based medium in a molecular state. However, the dye-based inks have a disadvantage that they fade earlier by ultraviolet rays or activated gas in the atmosphere. On the other hand, the pigment-based inks are excellent in resistance to fading when stored for a prolonged period of time.
In recent years, pigment-based inks are able to attain at the same time the original prolonged storage and the high color-development properties comparable to those of dye-based inks due to advancement of manufacturing technology. An inkjet printing apparatus which uses pigment-based inks has come to be widely used mainly in trademark print use such as photos and posters in which printed images are strongly requested to be stored over a longer period of time.
However, particularly in the above-described use in which pigments are used, there is found an ever increasing importance to the problem that the degree of gloss of an image tends to be nonuniform or to the problem of image quality that has been a concern, for example, a bronzing phenomenon typically found on the use of pigment cyan ink. Further, with an increase in exhibition use such as posters, there is posed a new problem on the fragility of fastness of a printed image which indicates the strength and prolonged storage of an image, as compared with offset printed matter.
Hereinafter, a description will be given, as an example, of a problem of scratch resistance among problems of fastness of a printed image.
There is now posed a problem that when pigment-based inks are mainly used to print an image on glossy paper, the image is likely to be damaged even when handling after printing or during display which is a general working step.
FIG. 4B is a schematic diagram illustrating the cross section of a printed image obtained when pigment-based inks are used to perform printing on a printing medium on which an ink receiving layer is formed. Hereinafter, a description will be given, with reference to FIG. 4B, of reasons for an image which is likely to be damaged when formed on glossy paper by using pigment-based inks.
A printing medium used in an inkjet printing apparatus is constituted so that an ink receiving layer 24 is formed on the surface thereof in order to absorb ink on a basic material (not illustrated) such as paper and film. The ink receiving layer 24 contains a great amount of inorganic fine particles such as silica and alumina high in absorbability of an ink solvent for the purpose of inhibiting ink spread or the like. Since printing media such as glossy paper used in printing a photo require a high surface smoothness, inorganic particles in the submicron range are generally used. Therefore, a clearance between inorganic fine particles formed on the ink receiving layer 24 is proportional to the particle size and formed with a fine pore in the submicron range.
On the other hand, a pigment-based ink 25 is an ink in which a color pigment is dispersed as particles of about 100 nanometers. Therefore, where the fine pores of the ink receiving layer 24 are smaller in diameter than color pigment particles, the color pigment particles are unable to enter into the ink receiving layer 24 and retained on the surface as if they were sieved. In general, in printing media such as glossy paper, fine pores of the ink receiving layer 24 are smaller in diameter than color pigment particles, and thereby the pigment-based ink layer 25 is formed on the surface of the ink receiving layer 24.
As described above, since the pigment-based ink layer 25 is formed on the surface of the ink receiving layer 24, the surface of an image is likely to be damaged on application of an external force to the pigment-based ink layer 25. The pigment-based ink layer 25 (image) may be peeled by the external force, depending on a case. Because of the above-described reasons, a problem of scratch resistance may be found dominantly on an image formed by using pigment-based inks.
In dealing with the above problem, it is quite effective in improving the scratch resistance by forming a transparent layer on the surface layer of the pigment-based ink layer 25 on glossy paper to decrease a coefficient of dynamic friction on the surface of an image. Thus, in an inkjet printing apparatus which has been developed recently, there has been proposed a constitution in which printing is performed with the use of glossy paper on which a treatment liquid containing a resin having functions of scratch resistance is used to form a transparent layer.
FIG. 4A is a schematic diagram illustrating the cross section of a printed image on which a treatment liquid is used to form a transparent layer. The treatment liquid is used to form a transparent layer 26 on the uppermost surface so as to coat the pigment-based ink layer 25. Since the pigment-based ink layer 25 is protected by the transparent layer 26, it is possible to obtain a printed image in which the image face is less likely to be peeled or damaged by an external force such as contact with a fingernail and also to improve the scratch resistance. However, the surface is not necessarily coated completely with the treatment liquid as shown in FIG. 4A. The scratch resistance may be improved, if the ink receiving layer is coated with the treatment liquid to such an extent that no external force is directly applied to pigment-based inks although some parts of the layer are not coated.
Japanese Patent Laid-Open No. H11-263052 (1999) has disclosed a method in which a film-formable transparent resin is heated, melted and ejected on an image from nozzles when inkjet printing, thereby coating the image with a transparent layer as a general method for forming the transparent layer 26 by using a treatment liquid. In this method, dots are arranged all over or uniformly on an image to form a transparent layer, thereby protecting the image.
Coating the uppermost surface of an image of pigment-based inks on a printing medium by using a transparent layer is quite effective in improving the scratch resistance and image performance such as gloss uniformity. However, the treatment liquid is applied to an image to be printed with plural colors of pigment-based inks on its whole part of the image, thereby resulting in a relatively greater amount of the treatment liquid compared with amounts of individual colors of pigment-based inks. As a result, such a problem is posed that a tank for a treatment liquid is made larger and the running cost is increased due to a greater consumption of the treatment liquid.